# How to get average of a function over each cell.

In FEniCS there are three different kinds of function space which we can use.

```
FS = FunctionSpace(mesh, "CG", 1)
VFS = VectorFunctionSpace(mesh, "CG", 1)
TFS = TensorFunctionSpace(mesh, "CG", 1)
```

As the name implies:

`FunctionSpace`

is to define scalar functions over the mesh and have one value per vertex.`VectorFunctionSpace`

is used to define vectors over the mesh and have \(d\) values over the vertex where \(d\) is the dimension of the problem. Thus in one dimension it will return a scalar, but it is necessary to define the quantities such as gradients on`VectorFunctionSpace`

which are vectors in 2D and 3D.`TensorFunctionSpace`

is used to define tensor functions over the mesh.

In this post I am going to talk about `FunctionSpace`

and `VectorFunctionSpace`

that I mostly use to define scalar or vector fields over the mesh. Suppose, I have the following mesh which is discretized with traingular elements haveing two degrees of freedom at each node.

This mesh is generate with the help of following commands

```
from dolfin import *
%matplotlib inline
mesh = UnitSquareMesh(1,1)
```

Next, we define the displacement vector field over the mesh. This could be achieved by first defining a `VectorFunctionSpace`

and then defining the `Function`

over the function space.

```
VFS = VectorFunctionSpace(mesh, "CG", 1)
unew, uold = Function(VFS), Function(VFS)
```

We can then manually define the vector field based on custom data. Under the hood - ignoring all the complex methods present in the function - a fucntion contains the values of the field at the degrees of freedom in the order

\[[x_0,y_0,x_1,y_1,x_2,y_2 ... , x_n, y_n]\]where \(n\) is the degree of freedom number. In the above mesh we have 2 degrees of freedom per vertex. Thus, we can modify the vectors

```
unew.vector().vec().array = np.array([0,0,0,0,1,0,2,0])
uold.vector().vec().array = np.array([0,0,0,0,0,0,0,0])
```

We can then find the error field over the mesh. Now, since we are finding the error between two vector fields the error will also be a vector field.

```
error_u = project(unew-uold,VFS)
```

The vector `error_u`

will contain two values at each vertex, one for error in x and other for error in y. We can check the values by using the method `compute_vertex_values`

```
error_u_x, error_u_y = error_u.split()
error_u_x.compute_vertex_values()
error_u_y.compute_vertex_values()
```

Now, I want the magnitude of the error at each vertex. For this I can write an expression in UFL for magnitude and then project that to a `FunctionSpace`

since I want a single value per vertex

```
FS = FunctionSpace(mesh, 'CG', 1)
error_u_mag = project(sqrt(inner(error_u,error_u)),FS)
```

This will give me a field containing the magnitude of the error at each vertex, now to average the error over each element I can take help of `Discontinuous Galerkin`

element

```
DG = FunctionSpace(mesh, "DG", 0)
error_u_mag_cell =interpolate(error_u_mag,DG)
error_u_mag_cell.vector()[:]
```

**Out:** array([ 1. , 0.33333333])

This way I will get the error magnitude over each cell in the mesh.